This invention generally relates to a composite for case bonding double base propellants and more particularly to a two layer composite for case bonding double base propellants.
The function of any case-bond system is to provide a reliable bond of the solid propellant to the case or the insulator over the range of service requirements for the motor. This joint is a point of stress concentration because of the wide dissimilarity in mechanical properties of the propellant and its substrate. Thus, rather than attempt direct coupling, case bonding is generally obtained by using some type of adhesive layer which also acts as the stress transfer medium. In double base work this material is referred to as a "case bond system" or a "bonding layer" while in composite work it is called a "liner". In this application the term "case bond system" as used is meant to refer to any double base propellant system.
Double base propellants use nitrocellulose, nitroglycerin and other plasticizers as the binder for conventional fuels and oxidizers. The ratio of plasticizer to polymer is generally very high. Usually a physical curing of the mass occurs by gelation of the nitrocellulose with the plasticizers.
At the time of casting when the propellant first contacts the substrate, separate liquid-solid phases exist but during the cure these phases progress to a gel and finally to a viscoelastic solid. The migratory solvating action which solidifies the mixture to a homogeneous mass also extends to adjacent materials. Even after the grain is apparently cured migration continues until equilibrium between all phases is attained. The diffusion of the plasticizer over a period of time into surrounding materials, such as the insulator, is a real problem unless a barrier is present to restrict migration and allow a quasi-steady state to be reached. Uncontrolled absorption of binder plasticizers by the case-bond system or the substrate can lead to complete loss of their mechanical properties, insulation features, adhesion or alter the ballistic properties of the motor.
The nitrate esters which form the propellant binder impose a chemical compatibility restriction and limit the materials which can be used in the presence of the propellant. Depending on the material, interactions can result in more or less rapid degradation of the bond interface. Products of incompatibility which are most serious from the standpoint of bond stability are gases, usually oxides of nitrogen.
Propellants must be bonded to a variety of substrates including elastomers such as SBR and EPT, asbestos-filled phenolic compositions, filament-wound epoxy-glass structures and various metals. In some instances more than one type may be included in the same motor. Thus, it is very difficult to obtain case bonding systems which will prevent nitroglycerin migration, be useful on many different substrates and operate efficiently over a wide temperature range and only a limited number of systems will work properly.